Laser transmission welding is generically described in German patent application DE 10 2007 042 739 A1. This type of welding has proven to be very practical and efficient for joining components, especially those made of plastic. It is based on the targeted application of at least one laser beam onto a contact zone in which two components touch each other, whereby the component facing the laser beam is transmissive for the wavelength of the laser beam employed, while the other component absorbs this wavelength. The laser beam passes through the transmissive component, strikes the surface of the absorptive component and heats it up. At an appropriate exposure time and power of the laser beam, the surface of the absorptive component melts. Heat conduction likewise causes melting of the areas of the transmissive component that are adjacent to the contact zone, creating a weld joint.
In order to achieve high-quality and reproducible welds, it is important for the components that are to be joined to be in close contact with each other at a prescribed contact pressure, at least along the areas that are to be welded, referred to below as the contact zone. This ensures an effective heat conduction and promotes the dimensional stability of the welded product, which is achieved in that an increasing counter-pressure to the swelling melt is generated.
A problem encountered here has to do with the holding and pressing together of the components at the contact zone, without hindering the effect of the laser beam. A number of solutions have been described for building up an appropriate contact pressure.
German patent application DE 10 2004 015 250 A1 describes a device in which the workpieces (referred to below as components) are pressed against a clamping roller in the area of the welding procedure by means of a clamping finger. This clamping device can press the components together at individual points. Welding using a vertically impinging laser beam can then be carried out in the vicinity of the clamping finger and the clamping roller. The clamping finger that grazes over the surface of the one component, however, also entails the risk of damage.
In order to be able to apply clamping forces onto a contact zone as vertically as possible and, at the same time, in order not to block a laser beam that is directed at the contact zone, European patent application EP 1 405 713 B1 describes the selective use of a transparent ball or a transparent roller that is moved under pressure over the surface of the component (here the workpiece) made of the transmissive material, as a result of which the components below the placement point of the ball or roller are pressed against each other. The position of the workpieces that are to be joined can be fixed by creating punctual welds before the beginning of the actual joining procedure. The laser beam is passed through the ball, a process in which the ball or roller functions as a focusing lens.
The two above-mentioned solutions have in common the fact that a contact pressure is exerted along the contact zone so as to act only punctually upon said contact zone; that is to say, the contact pressure only acts directly at the place where the laser beam strikes. These devices are thus only suitable for contour welding in which the welding takes place with the a one-time pass of the laser beam over the contact zone.
Therefore, the use of efficient methods such as simultaneous welding or quasi-simultaneous welding in which the entire contact zone is welded simultaneously or quasi-simultaneously is not possible with those solutions. Simultaneous welding and quasi-simultaneous welding require contacting under the effect of contact pressures along the entire contact zone.
Moreover, in order to be able to weld a ring-shaped contact zone, if a laser scanner is used, it should be positioned above and advantageously in the center area of the ring-shaped contact zone. From such a position, the contact zone can be scanned especially effectively by the laser scanner.
Therefore, devices or their parts, which are likewise above the contact zone and are in contact with the components, can hinder the scanning procedure. Using transmissive materials such as, for example, glass plates as pressure elements, and transmitting the laser beam through them entails the risk that impurities will become burned into the transmissive materials.
If the contact zone extends to the edges of the components, it is difficult to use so-called external clamping devices, which apply a contact pressure from the edges of the components. Possible alternatives involve the use of suitable internal clamping devices.
German patent application DE 10 2007 042 739 A1 describes a clamping device for exerting a contact pressure on two components along a shared contact zone in order to join the two components employing the laser transmission welding method. The two components in question here are, on the one hand, a quadrangular open lower component and, on the other hand, a cover that is placed on top of the lower component and that closes flush with the outsides of the lower component. Together, the surfaces that touch each other form a contact zone, which here has a rectangular, flat, continuous ring-shaped form along which the two components are welded together.
A problem encountered with welding contact zones having a flat, continuous ring-shaped form using a laser beam includes the fact that the laser beam and the requisite contact pressure should act on the components from the same direction, while the mechanical means with which a contact pressure is transmitted to the components should not block the laser beam.
According to German patent application DE 10 2007 042 739 A1, the clamping device comprises a receptacle to support the lower component, an internal clamping chuck which, on the one hand, lies against the cover inside the contact zone and, on the other hand, is in contact with the contact zone in such a way that it projects over a clamping head (referred to below as a pressure element) whose open inside area is larger than the contact zone and from which a contact pressure acts on the cover via the internal clamping chuck.
In order for a laser beam coming from a laser welding head to be able to strike the cover along the contact zone, an encircling slot that has the shape and size of the contact zone and that is interrupted by connecting struts is incorporated into the internal clamping chuck.
A special shaping of the connecting struts should keep the blocking effect as small as possible in order to ensure that the weld is of a sufficiently good quality. In a special embodiment, the pressure elements can also be swiveled out of the path of the laser beam. However, such punctual alleviations of the load result in unequal clamping conditions over the contact zone. Fundamentally, a clamping device according to German patent application DE 10 2007 042 739 A1 would probably be equally suitable for use in contour welding, simultaneous welding and quasi-simultaneous welding.
There are limits to the reduction of the number and width of the connecting struts that block the laser beam, since these struts are instrumental for ensuring that the force transmission onto the cover is uniformly distributed and as complete as possible. The connecting struts and thus the entire internal pressure punch should be made of a material with a high bending stiffness.
Because the properties of a weld that is being created in the contact zone area below the connecting struts in which the direct effect of the laser beam is permanently blocked differ from those of the weld in the slot areas and, no truly homogeneous weld can be created along the contact zone.